Electrical measuring circuit



Patented v'June 20, v1939 ELECTRICAL" MEASURING CIRCUIT Alphons V.Wurmser, West Englewood, N. J., assgnor to Bell lTelephone Laboratories,Incorporated, New York, N. Y., a corporation of New York Applicationseptember 2,1937, serial No. 162,113

Claims.

This invention relates to electrical measuring circuits and moreparticularly to means for measuring the electrical power present in acircuit by a single measuring unit whether this 5 power be in the formof simple sine wavedisplacements or in the form of complex wavedisplacements, as for noise on telephone lines.

In the case of a line on which there is impressed a pure sine waveelectromotive force the .0 power flowing in the line is proportional tothe square of the amplitude. If two such waves of the same frequency andof equal amplitude are impressed in phase with each other the resultantis a single wave of double the amplitude and l5 four times the power. Ifthe two waves are not m phase the resultant is still a pure sine wavebut of an amplitude dependent on the phase angle between the componentsand the power is proportional to the square of ytlie amplitude of lo theresultant. -If the two components are of incommensurable frequencies thepower is the sum of the power due to the individual waves. So also forany complex wave'made up of noncoherent components, as for noise.

Ordinarily, for alternating current circuits it is customary in takingmeasurements to have the voltage across a transmission line or analogouscircuit or the current flowing therein limpressed l on some device suchas a rectifier with a meter in its output operating to give a directcurrent reading in response to the voltagev or current. The device maybe so designed as to give a reading proportional to said voltage orcurrent or a read'- ing proportional to some power thereof. In theformer case if two waves of thesame frequency and amplitude and in phasewith each other are present, the reading will be greater than for one ofthe waves bythe factor 2 but the power will be four times as great. Ifthe two waves, how- 40 ever, are of incommensurable frequency then thereading on the responsive device will be greater than forl one by thefactor On the other hand, if a device is used in which the reading isproportional to the square of the Y impressed voltage, as in the case ofthermocouples and various other responsive devices, then 5d againvthe'reading of the instrument will be dii-v ferent, depending uponwhether the two components are'of thev'same or of incommensurable frequencies. For the case. of equal 'frequencies in phase the readingyfor two waves will be four times that for one wave whereas forincommensurable frequencies the reading will be twice that for one wave.

, Recently I have devised a linear amplierrectifier described in mycopending application, Case 2, Serial No. 131,190 led March 16, 1937,now Patent 2,147,729, issued February 21, 1939, comprising an amplifierand a rectifier in tandem. Even though the amplifier and the rectier arethemselves not strictly linear, such a circuit combination may berendered nearly linear by insertion of resistances which are quite largecompared to the impedance of the amplifier or the rectifier units. Suchresistances, of course, introduce large losses and thus greatly decreasethe sensitivity of the measuring circuit. If these resistances areremoved, then such circuitslose their linearity and at the same timeI-find them to be unstable d'ue to changes in temperature and otherirregularities. 'I'his I have largely overcome, as described in mycopending application,

' by a negative feed-back connection from the output to the input of theamplifier-rectifier combination to adjust its gain in accordance withthe output level, which greatly increases' the stability of the circuitin a manner characteristic of feedback circuits permitting the omissionof at least most of the resistance and making it possible to Vobtainreliable readings on the transmission,-

characteristics of a line of a high degree of accuracy.. Such a circuitis linear, its response for single frequency being proportional to theinput voltage (or currents), and the recording or in' dicatinginstrument may be anammeter with a j uniform scale.

sion lines it has been the necessary practice to determine theinterference on a power basis using thermocouples or'other devices forwhich the output follows quite closely-the square law. 'I'he recordinginstrument in this case is also usually calibrated on a power basis. Itis desirable lin practical telephone engineering that the same recordershall -be suitable for measuring both of the types ofveffects heretoforediscussed. How-v ever, if the linear amplifier-rectifier is calibratedso that for single frequency input the direct current output is`proportional to the alternating current input, then for a complex waveof incommensurable frequencies the reading may depart some 10 or 15 percent from that indicated by a ltherrnocouple and if the wave is made upof a fundamental and its harmonics the error may be substantiallygreater, the instrument always underestimating the actual power. It isthe purpose of this invention to devise a measuring cir- In measurementsof noise eects on transmiscontrol grid-cathode circuit.

cuit in which a single recording or indicating instrument with a singlecalibration shall be equally applicable for measuring both kinds ofeffects.

The invention will be better understood by reference to the followingspecification and the accompanying drawing, in which:

Fig. 1 is a circuit diagram of my measuring set;

Figs. 2 to 6 are curves characteristic of the different parts of thecircuit; and

Fig. '7 is a circuit diagram showing one Way of using this invention,

Referring more particularly to Fig. 1 there is shown an inputtransformer 4 the primary of which is connected to the line on whichmeasurements are desired. Connected with the secondary of thetransformer 4 is the input of an amplifier 6 the output of which,through the transformer 8, is impressed on a recter circuit to give adirect current reading in a meter I0. The rectifier circuit may take avariety of forms but in that form shown in Fig. 1 it consists of adivided secondary 9 each half of which is connected through a rectifyingelement l l across the common meter l0. These rectifying elements may beof any suitable form but one whichI nd especially convenient is thatknown as the copper-oxide rectifier.

The amplier 6 may employ any one of a large number of types of amplifiertubes adapted for operation to give a desired characteristic to bedescribed below, the one here shown being of the pentode type. The tubeis rendered self-biasing by the resistance I3 with capacity shunt I4 inits The impedance of the output circuit of this tube is also renderedvariable by means of the variable resistance I6 in series in theplate-cathode circuit of the tube and the characteristic of the tubecircuit may then be altered Within reasonable limits. In the rectifiercircuit there is included in series with the meter l an adjustableresistance Ilv which may be used to modify to some extent the overallcharacteristic of the rectifier circuit.

It is characteristic of practically all rectifying devices that thecurrent output is not strictly proportional to the impressed voltage butis nonlinear, as illustrated by the curve of Fig. 3, indicating that theycurrent Ir increases more rapidly than the impressed voltage E, 'Iherelationship may be of the form Ir=k1cm where m may be larger or smallerthan unity, although-l more commonly the former. If the exponent has thevalue 2 then theresponse of th'e meter in the rectifier output isproportional to the power im pressed on the recfler circuit. If,however, 1t is desired that the meter have a deflection which isproportional to the impressed voltage on a single frequency basis thensome compensation must be made. This may be accomplished in the circuitof the invention shown in Fig. l by choosing the tube 6 and adjustingits circuit constants so that its characteristic is complementary tothat of the rectifier circuit. In this event the plate currentcontrolgrid voltage characteristic of the tube 6 and its associated circuit maybe shown by the curve of Fig. 2, which it will be noted would berepresented by an expression of the form Ip=k2en in which n hat a valueless than unity, It will be noted that under these conditions the tube 6and its circuit has the characteristic of a compressor, that is theamplification for large input is not as great as that for small input.On the other hand, it will be noted from Fig. 3 that the characteristicof the rectifier circuit is that of an expandor. If the characteristicsare so adjusted that then the compression effect of the one isneutralized by the expansion effect of the other and l the circuit islinear in its response. Such a combination gives what may be calledalinear amplifier-rectifier circuit and this linearity is illustratedmore fully in Figs. 4 and 5. In Fig. 5, for example, the gain of therectifier (which in this case is a negative gain) is plotted in decibelsagainst the input voltage and if the coefficient m is constant, the gain`characteristic will be a sloping straight line, as shown bycurve a.Similarly, if the exponent n is constant, then the gain characteristicof the amplifier circuit is a sloping straight line as shown by thecurve of Fig. 4. If

then the over-all characteristic of the circuit is represented by thebroken line curve b o f Fig. 5, showing a gain which is independent ofthe impressed voltage Such a relationship of characteristics may beobtained over a considerable range.

Now the introduction of compression at one point followed by an equalexpansion would, in and of itself, yield a device with an over-alllinear characteristic and for a single frequency input the output wouldbe linearly proportional to the input amplitude, which is desirable formeasurements on the transmission characteristics of a line. It would notbe suitable for noise measurements which should be made on a powerbasis. I have discovered, however, contrary to expectations, that such acircuit may be adjusted to read on a power basis for both a singlefrequency wave and a complex wave. It will be observed from Fig. 2 thatthe single tube compressor of Fig. 1 is itself non-linear and sointroduces distortion. The circuit constants are adjusted so that a puresine Wave will produce in the output of the pentode tube amplifier alopsided wave, the loops on transformer 8 eliminates any direct currentcomponent resulting from this distortion but nevertheless passes on awave modified by the distortion due to the amplifier.

I have discovered that it is this property which allows the circuit as awhole to be used for measuring accurately on a power basis both forsingle frequency and complex impressed waves over a considerable range,with the added feature that themeter reading is linear with respect tothe root mean square for both cases. The proper ratio of theampliications of the positive and negative portions of the amplifiedwave to provide accurate measurements for both types of input isobtained by suitable adjustment of the circuit constants of the pentodetube 6, for example, by making the voltage on the plate of the tube ofthe required value to accomplish this result by adjustment of thevariable resistance I6. As indicated above, the characteristic of therectifier circuit may be modified somewhat to assist in making itscharacteristic complementary to that of the amplifier 6 by adjusting thevariable resistance Il. A relatively simple and favorable condition isobtained when We have a dissymmetrical square root law amplifiercombined with a full wave symmetrical square law rectifier but thepeculiar property of the circuit referred to above is not restricted tosuch characteristics.

The kind of distorted wave which is passed through the transformer 8 tothe rectifier in the system of Fig, 1 is indicated in one specic case byFig. 6, which is an oscillograph record of the rectified current i'n themeter of the rectifier circuit with a sine wave of 100 cycles impressedon my Patent No. 2,147,729, and the circuit of this invention inparallel, but using alternately the same reading instrument,v as shownin Fig.` 'l Where, in block form, the mst-mentioned circuit is shown atI8 and the circuit of this invention at I9. By a suitable switchingmeans 20 the meter I0 may be connected to I8 or I9, the one calibrationscale serving for both circuits.

While the description thus far has implied that the recording instrumentor meter shall be one in which the deflection is proportional to avoltage (or a current), this is not necessary. The instrument may becalibrated on any basis desired. For example, in many cases it would beconvenient to have it calibrated' on a decibel basis. In any case,however, 4the instrument would still be appropriate with its one scalefor both circuits giving readings, when connected with the circuit I8,which vare proportional to the amplitude of the single frequency waveused for transmission measurements and giving readings when connectedwith the circuit I9 proportional to the root mean square of the powerpresent on the line, whether this power refersto that of asinglefrequency o r a. complex wave.

What is claimed is:

1. In a circuit for measuring the amplitude of a.l single frequency waveor the root mean square of -a complex wave, an amplifier anda rectifierin tandem, a meter in the rectifier circuit, the amplier circuitcharacteristic being non-linear and the rectifier circuit characteristicbeing nonlinear in a complementary manner, and a. transformer couplingthe amplier circuit and the rectifier circuit. i 4

2. In a measuring circuit for alternating current wave eifects, anamplifier circuit with an exponential characteristic, a rectifiercircuit with an exponential characteristic, the exponent for the latterbeing-the reciprocal of that for the amplifier circuit, a transformercoupling between the two, and a meter associated with the rectifiercircuit.

3. A circuit for accurately measuringthe .volume level of singlefrequency or complex waves over a wide range of volumes comprising anamplier-rectifier in which the compression characteristic of theamplifier is substantially complementary to the expansion characteristicof the rectier, means to impress the wave to be measured on the input ofsaid ampliner-rectiiier,

means to cause said amplifier to produce a,l different amplification forthe positive portion of the vapplied wave than for the negative portionthereof in the proper ratio to provide correct power addition, means tosuppress from said rectifler the direct current component introduced bythe dinerent amplification of the two portions of said wave, and asingle linear decibel meter for indicating the level of the rectiedcurrent in the output of said ampliiier-rectiner.

4. A measuring circuit comprising apdissymmetrica] square root lawamplifier, a full-wave symmetrical square law rectifier, a transformercoupling the output of said amplifier to the input of said rectifier anda meter for indicating the rectifier output.

5. A circuit for measuring the amplitude level of alternating currentwaves comprising a pen`v` -tode vacuum tube for amplifying said waves, a

symmetrical full-wave rectifier having an expander characteristic forrectifying the amplified waves, a transformer coupling the output of'said vacuum tube to the input of said rectifier, means for controllingthe output impedance of said tube so that it provides unequalamplifica.- tion of the positiveand negative portions of the impressedwaves in a desired ratio, and has a compressor characteristic which issubstantially complementary to the expander characteristic of saidrectifier for a substantial range of input level,

and a meter for indicating the rectiiier output.

ALPHONS V. WURMSER.

